The NAD-dependent ligase encoded by yerG is an essential gene of Bacillus subtilis

DNA ligases are grouped into two families, ATP-dependent and NAD-dependent, according to the cofactor required for their activity. A surprising capabi lity of both kinds of ligases to complement for one another in vivo has bee n observed. Bacillus subtilis harbours one NAD-dependent ligase, YerG, and two ATP-dependent ligases, YkoU and YoqV, this last one being encoded by th e 134 kb lysogenic bacteriophage SP beta and consisting of a single adenyla tion domain typical of ATP-dependent ligases, Because the genetics of ligas es in B.subtilis had not been studied previously, the genes encoding for on e ligase of each kind, yerG and yoqV, were investigated. We found that the yerG gene was essential in B.subtilis, This suggests that none of the ATP-d ependent ligases was able to complement the yerG defect, In addition, the A TP-dependent ligase encoded by yoqV, when cloned on a plasmid under appropr iate expression signals, was unable to rescue a yerG mutant strain. The two B.subtilis ligase genes yerG and yoqV were also introduced in an Escherich ia coli strain encoding a thermosensitive ligase (ligts), and whereas yoqV did not complement the ligts defects, yerG fully complemented the growth an d UV sensitivity defects of the lig mutant. We propose to rename the yerG a nd yoqV genes of B.subtilis ligA and ligB respectively.